Use of polysilicon by the photovoltaic industry has been growing rapidly and in 2005 this demand was essentially equivalent to the use of polysilicon by the microelectronic industry. The anticipated growth rate of the photovoltaic industry is expected to be between 15 to 30% (recent year growth has been at 30 to 45%) compared to the microelectronic industry at 7 to 10% which will result in much larger demand of polysilicon for the photovoltaic industry. While the silicon wafer cost constitutes approximately 25 to 33% of the PV (photovoltaic) module costs, it is less than 5% of the silicon semiconductor device costs in the microelectronic industry. Therefore, it is imperative to reduce the cost contributions of polysilicon for the photovoltaic industry. The PV industry has learned to use polysilicon with minor imperfections and slight contamination as one way to contain costs.
One of the most widely practiced conventional methods of polysilicon production is by depositing polysilicon in a chemical vapor deposition (CVD) reactor, and is referred to as Siemens method. Referring to prior art FIG. 1, a CVD reactor consists of a base plate 23, and a chamber wall or quartz bell jar 24. There is incorporated in base plate 23, a gas inlet 20 and a gas outlet 21 (can be in the same position), and electrical feedthroughs 19. A viewing port 22 provides for visual inspection of the interior or for the temperature measurement.
In the prior art polysilicon manufacturing by CVD, a high-purity silicon slim rod structure or filament is assembled in the form of a hair pin by having a cross rod 2 placed horizontally on two long, spaced apart, vertical rods 1 and 3. This structure is mounted and connected so as to provide a current path between electrical feedthroughs 19. During the CVD process, polysilicon deposit accumulates uniformly on the slim rods; the deposit 41 being shown here partially removed to show the slim rod structure. Different users employ different methods for joining the horizontal rod to the vertical rods. One method requires a groove or a key slot at the top of each vertical rod. A small counter bore or conforming fitment is formed at the ends of the horizontal rod so that it can be press fitted into the grooves to bridge the two vertical rods.
Because of the high purity silicon from which these rods are fabricated, the corresponding electrical resistance of the slim rods is extremely high. Thus it is extremely difficult to heat this silicon “filament” using electrical current, during the startup phase of the process.
Sometimes the slim rods are replaced by metallic rods that are more conductive and easier to heat with electrical current. This method is referred to as Rogers Heitz method. However, the introduction of metal into the chemical vapor deposition process can introduce metal contamination. This contamination of the polysilicon yield is not acceptable in the semiconductor/microelectronics industry. However, for the photovoltaic industry the wafers used for fabricating solar cells are typically doped with Periodic Table group 3 elements, such as boron (B), or group 5 elements, such as phosphorous (P), to make them more conductive.
Resistivity of pure silicon is a strong function of temperature, ranging from 106 ohm·cm for a slim rod at room temperature to 0.01 ohm·cm at 1200 deg C. Doped silicon, however shows a different behavior. Depending on the concentration of the dopant, e.g, Boron, the resistivity will increase along with the temperature to a certain point, and then become the same as an intrinsic silicon slim rod. At room temperature, a boron doped silicon slim rod at 1018 atom/cm3 is about 0.05 ohm·cm. There is some tolerance for impurities, especially for the dopant ions, when polysilicon is used for photovoltaic applications.
A typical prior art reactor for conducting a Siemens-type process includes a complex array of subsystems. External heaters are used to raise the temperature of the high purity slim rod filaments to approximately 400° C. (centigrade) in order to reduce their electrical resistivity or impedance to current flow. Sometimes external heating is applied in form of halogen heating or plasma discharge heating. Normally, a multi-tap electrical power supply is required for the resistance heating of the filaments. It can provide very high voltages and low current for the early phase heating; and a very high current at relatively lower voltage for the later phase when the resistivity of the rods has been decreased by the higher temperature.
High voltage switching gear is needed for switching between the power level taps. The first process of sending low current at high voltage through the filaments continues until the temperature of the filaments reaches about 800° C. At this temperature, the resistance of the high purity silicon rods falls very drastically and the high voltage source is switched to the low voltage source that is capable of supplying the high current. However, since the current drawn by the silicon slim rods at around 800° C. is of a run away nature, the switching of the high voltage to low voltage power source needs to be done with extreme care and caution.
During the CVD process, silicon deposits onto the hot surface of the filaments and the diameter of the resulting silicon rods becomes larger and larger. Under the constant process conditions of gas supply, reactor pressure, the surface temperature of the growing rods (typically 1100 degrees C. for using trichlorosilane as the decomposition gas, for example), the rate of the diameter increase (or the deposition rate in terms of micrometer per minute) is more or less constant. The typical starting size of the silicon slim rods is about 7 mm with a round or square cross section. The size of the metal wire slim rods is even smaller. Therefore, the production rate in terms of kg per hour is very low at the initial stage when the silicon rod diameter is small.
In one kind of conventional CVD reactor, high purity silicon is deposited by reaction of trichlorosilane (SiHCl3) and hydrogen (H2) onto solid slim rods of typically 7 mm diameter. In a typical reactor an array of slim rods are assembled; this placement is based on radiant heat transfer between the rods, heat losses to the outside wall and deposition rate of silicon on these slim rods. Faster deposition rates can result in imperfections in polysilicon productions which is not acceptable to the microelectronics industry; however the photovoltaic industry has learned to deal with such minor imperfections.
There have been past efforts made to modifying the current CVD reactors with the intent to simplify the number of slim rods or to increase the deposition rates, but they have not achieved widespread acceptance as the new reactors deviated considerably from the conventional reactor designs and it would be very costly and time consuming to retrofit or replace existing CVD reactors and optimize all other parameters prior to commercialization.